1. Field of the Invention
The present invention relates to a frame for a motorcycle. More particularly, the present invention relates to a motorcycle frame of a mono-backbone type, in which a single main frame extends rearwardly along a center of vehicle width, from a head pipe to a pivot portion.
2. Background Art
There is a conventional frame assembly structure in which a main frame, for suspending an engine, extends rearwardly from a head pipe along the center of vehicle width, and then bends on the rear side of the engine so as to extend downwardly to a pivot portion. In this conventional structure, left and right side frames are fastened to a boss, provided in the vicinity of the pivot portion, and a front end portion of a swing arm is held between the main frame and the left and right side plates. Such a frame assembly structure is disclosed, for example, in Japanese Patent Laid-Open No. Hei 09-095279.
The boss, which permits adjustment of the spacing between the pivot portion and the left and right side plates, is provided in the vicinity of the pivot portion, and, in the vicinity of the boss, there is provided a boss for fastening an engine hanger.
In the above-described conventional structure, however, when modifying the swing arm width, due to a modification in the vehicle body design, the adjustment of the spacing between the main frame and each of the side plates must be carried out by modifying the main frame, which is a comparatively large component part. This leads to an increase in cost, and imposes a limitation on the degree of freedom in vehicle body design.
In addition, in the above-described conventional structure, joint surfaces of the bosses that abut on the side plates, used for fastening the side plates, and a joint surface of the boss that abuts on the engine hanger, used for fastening the engine hanger, are provided at different heights. Therefore, when carrying out mechanical work to enhance the surface accuracy of each of the abutting joint surfaces or other similar case, the mechanical work processes are conducted individually, resulting in an increase in the number of steps for manufacturing the frame assembly.
The present invention has been made in consideration of the above-mentioned circumstances, and provides a motorcycle frame in which spacing between a main frame and each side plate can be easily adjusted, and in which joint surfaces, abutting on the side plates and an engine hanger, of the main frame, can be easily processed.